Generally, as for filter circuits, such as a BPF (Band Pass Filter) used for wireless communication devices, very high accuracy is requested in frequency characteristics. Then, in order to prevent frequency characteristics from shifting from desired characteristics by a shift of resistance or capacitance resulting from variations in elements when incorporating a filter circuit in an IC (integrated circuit), an automatic regulator of a filter is used.
The automatic regulator of a filter is constructed by providing a dummy filter circuit (dummy filter) in addition to a filter circuit (main filter) which performs original filtering processing to an input signal (e.g., refer to patent document 1). Although a BPF constructs a dummy filter in this patent document 1, there is also the one which uses a VCF (Voltage Controlled Filter), which is constructed of a secondary LPF (Low-Pass Filter), as the dummy filter.
Patent document 1: Japanese Patent Laid-Open No. 9-98446
FIG. 1 is a diagram showing a conventional automatic regulator of a filter. In FIG. 1, reference numeral 101 denotes a main filter, which performs original filtering processing to an input signal. Reference numeral 102 denotes a dummy filter, which performs filtering processing to a reference signal Fr of a reference frequency. This dummy filter 102 is constructed in the same form as that of the main filter 101. For example, when the main filter 101 is constructed of a Butterworth filter, the dummy filter 102 is also done of a Butterworth filter, or when the main filter 101 is constructed of a Chebyshev filter, the dummy filter 102 is also done of a Chebyshev filter.
Reference numeral 103 denotes a phase comparator, which detects a phase difference between the reference signal Fr and a signal outputted from the dummy filter 102, and outputs a phase error signal according to the phase difference. Generally, as for a secondary filter, a phase of an output signal changes from 0° to 180° according to a change of an input frequency. Since a secondary filter is used for the dummy filter 102, if a cut-off frequency of the dummy filter 102 does not shift from a desired value (if it is in an ideal state), an output of the phase comparator 103 is converged in a state that a phase difference shifts by 90°. However, when the cut-off frequency of the dummy filter 102 shifts from the desired value by variations in elements, etc., the phase difference of the output of the phase comparator 103 becomes 90°+Δθ. The phase comparator 103 outputs a phase error signal for the error phase equivalent to this shift Δθ.
Reference numeral 104 denotes a loop filter, which is constructed of a parallel circuit of a resistor R and a capacitor C. This loop filter 104 outputs a signal proportional to the error phase detected by the phase comparator 103 by discharging and charging electric charges of the capacitor C on the basis of the phase error signal outputted from the phase comparator 103. Reference numeral 105 denotes an operational amplifier, which is inserted between the loop filter 104 and dummy filter 102 in order to raise a loop gain. The signal outputted from the loop filter 104 is fed back to the dummy filter 102 as a control voltage for the dummy filter 102 through the operational amplifier 105.
As described above, while the reference signal Fr of the reference frequency is inputted into the dummy filter 102 and phase comparator 103, an output of the dummy filter 102 is inputted into the phase comparator 103. Then, the output of the phase comparator 103 is inputted into the dummy filter 102 through the loop filter 104 and operational amplifier 105 as a control voltage. As the result, the dummy filter 102 is regulated, automatically. That is, even if the cut-off frequency of the dummy filter 102 shifts from the desired value by the variations in elements, etc. (even if the phase difference of the output of the phase comparator 103 shifts from 90°), it is regulated so that the phase difference may be 90° by a control loop using the phase error signal outputted from the phase comparator 103.
As described above, the main filter 101 is constructed in the same form as that of the dummy filter 102. Thereby, it is possible to perform an automatic regulation of the main filter 101 by inputting the control voltage for the dummy filter 102 also as a control voltage for the main filter 101. Namely, even if the cut-off frequency of the main filter 101 shifts from the desired value by the variations in elements, etc. (even if a phase of an output signal shifts from 90°), it is regulated by the phase error signal outputted from the phase comparator 103 so that the phase of the output signal of the main filter 101 may be 90°.